Guardian Angel DF RFI
AFLCMC issued RFI FA8629-25-R-5053 for a man-portable direction finding system to support Combat Search and Rescue teams, also known as Guardian Angel. Posted 26 Aug 2025. Due 26 Sep 2025.
https://sam.gov/opp/cba049ce33c740ada649b58e3a617bde/view
Who they are and why this matters
Guardian Angel is the Air Force’s dedicated personnel recovery force. Built around Combat Rescue Officers, Pararescue, and SERE Specialists, these teams prepare personnel for isolation, execute recoveries in peacetime and war, and lead reintegration afterward. They are the ground element of the rescue triad that commonly works with HH-60 helicopters and HC-130 aircraft.
What the Air Force is asking for
Provide a portable DF system that turns a survivor’s transmission into a usable location quickly. Push bearing, position, time, and confidence into ATAK so ground and air see the same picture. Log the RF trail. Interrogate when authorized. Lasts more than 8 hours on a single charge. Survive rain, shock, vibration, and temperature swings. Keep the kit under 12 pounds. Targets include about 5 degrees average bearing error at threshold with a 2 degree objective, coverage from 500 kHz to 6 GHz, detection of the Combat Survivor Evader Locator (CSEL) at 250 meters with a 16 kilometer objective, and geolocation accuracy of 10 meters at threshold with a 1 meter objective. Size is capped at roughly 3.7 × 7.4 × 8.8 inches at threshold, with an objective of 6 × 3 × 2 inches.
The anchor for the requirement
CSEL, the Combat Survivor Evader Locator, is the survival radio already issued to U.S. aircrew and other at-risk personnel. It provides UHF line-of-sight voice and data, as well as a 406 MHz distress burst for the international satellite rescue system. Newer sets also include SATCOM modes. It embeds GPS and a unique ID, and supports challenge-and-response interrogation, allowing recovery forces to authenticate the survivor in the field. If your DF kit reliably acquires CSEL, keeps bearings steady while the operator moves, and authenticates without pausing DF, you have addressed the core CSAR use case.
The hard problems that actually matter
Accuracy versus aperture: With a 12-inch head and solid calibration, 5 degrees is realistic in VHF and UHF. Achieving 2 degrees is possible in parts of the band when SNR and dwell are favorable or when bearings are fused over movement. Two degrees across 500 kHz to 6 GHz in a pocket head is not realistic. As the team closes on the source, far-field assumptions break. A close-in mode with a near-field model and faster updates is necessary to maintain steady bearing in the final 50 to 100 meters.
Full frequency coverage: Receiving 500 kHz with a small loop is straightforward. Tight bearings there are not. Treat sub-30 MHz through HF as detect and triage. Put precision DF from about 30 MHz to 6 GHz on a compact circular array. Use banded preselection and maintain clean dynamic range, or intermodulation from strong broadcast or tactical emitters will smear bearings.
CSEL range, honestly: The 250 meter threshold is trivial. The 16 kilometer objective is achievable with elevation or added gain in clean terrain, but not as a routine ground-to-ground result at body height in clutter. Line of sight and antenna height dominate at UHF and 406 MHz. Suppose you show 10 to 16 kilometers, state the terrain and geometry. Keep DF running during interrogation and present confirmation clearly so the team can act without second guessing.
Frequency hopping and concurrency: Hop sets cannot break the track. Channelize the watched span to keep end-to-end latency around 50 to 100 ms, allowing an operator to walk the line without overshoot, and preserve bearing continuity across hops. Multi-emitter means simultaneous capture and processing, not time slicing. Let the operator pin one target while background signals remain labeled and out of the way.
Geolocation, precisely: A single DF unit outputs bearings. 10 meter accuracy is realistic by fusing bearings over movement with decent geometry. One meter is only achieved when you ingest authenticated GPS from CSEL after authorized interrogation or when two synchronized nodes provide TDOA. You should say that outright.
Runtime inside a sealed box: More than 8 hours must include continuous DF, logging, and live ATAK updates. Sealed rugged enclosures trap heat. If the thermal path is weak, the processor will throttle, and the runtime claim falls apart. To approach 16 hours, use hot-swap packs or duty cycling that does not drop the track.
Ruggedization without losing phase: Passing IPX labels is one thing. Holding phase after drops and temperature swings is the real test. Solve array rigidity and oscillator stability. Provide a fast field auto-cal that the operator can run in seconds after a drop or a heat soak.
ATAK performance: Compatibility is not the bar. Bearings, position, time, confidence, and a breadcrumb must flow into Cursor on Target at a rate a human can move on. The plugin must be native and stable so that HH-60 and HC-130 crews see the same picture without needing babysitting.
Security that survives contact: Use secure boot, signed firmware, encrypted storage and links, mutual authentication on data exchange, and a fast zeroize that clears keys and mission data. Recovery gear cannot leak information.
Architecture, trade space, and upgrade path
Split the RF path. Use a small loop for 500 kHz to HF when needed. Use a compact circular array for frequencies ranging from roughly 30 MHz to 6 GHz, where precision is required. A single brick is simple to carry but forces a small aperture and tight thermal margins. A small head cabled to compute spreads heat and allows a stiffer array at the cost of a cable. A pure handheld is fast to deploy, but caps bearing precision in VHF. A small backpack head can add elements and stiffness for better UHF and L-band performance. Two synchronized nodes unlock TDOA and true 1 meter geolocation. Be explicit about what you fix and what you trade.
What to show so evaluators believe you
Show accuracy versus temperature with the actual head on a turntable. Then drop it and show the same plot. Provide hop continuity logs that show no bearing discontinuity at realistic dwell. Run multi-emitter tests and state when bearings or the UI degrade, along with the count at which this occurs. Publish power draw and internal temperature over a full shift with logging and ATAK connected. Run open, suburban, and urban lanes and show time to 10 meters by fusing bearings over movement. If you demo 16 kilometers, include terrain and antenna heights. Offer to deliver two pilot units in six months as the RFI requests.
Mission vignette
A Guardian Angel team inserts at night into broken terrain. A faint CSEL distress burst is detected at UHF. The DF kit locks quickly and posts a bearing with 70% confidence into ATAK, visible to the team and the HH-60 overhead. As the point man moves along a ridgeline, bearings fuse and the error ellipse shrinks from hundreds of meters to tens. The signal hops, but the bearing never resets. Other emitters are present but remain labeled in the background. When cleared, the operator interrogates the CSEL. The radio responds with authenticated GPS, which posts into ATAK while bearings continue updating. The team and the aircraft see confirmation at the same moment. After five hours in cold rain, the unit is stable, calibrated, and accurate. On the final approach, close-in mode engages, snapshot rate increases, the near-field model holds, and the team closes the last 80 meters without wobble. That is the difference between searching in the dark and executing recovery with precision.
Closing thoughts
This requirement asks teams to combine sound RF engineering, proven survival radios, and the operational needs of Guardian Angel into a system that works under stress. Thresholds are achievable today with honest design choices in aperture, power, and ruggedization. Objectives are stretch goals, some conditional, some geometry dependent, and some that require modular upgrades. A credible response will show what works now, what improves with motion or team geometry, and what needs deliberate development. The teams that bring accuracy plots, hop logs, ATAK feeds, and runtime under load will shape how this RFI matures into a program. Guardian Angel needs tools that turn survival radios into locations quickly. The teams that prove it with clarity and data will stand out.